The present invention relates to a dry method of preparing cationic starch ethers by reacting starch with alkylene epoxides in the presence of water in an alkaline medium.
The reaction of starch or of substances containing starch in so-called dry methods with cationic etherification agents is known. In these methods the reaction is not carried out in an aqueous suspension but rather with a starch which appears dry on the surface. According to U.S. Pat. No. 4,127,563, alkylene halohydrines are used for the cationization at a pH of less than or equal to 9 and a temperature of 25.degree. to 60.degree. C.
The reaction of a halohydrin with starch in an alkaline medium but at a temperature which is at least 30.degree. C. under the gelation temperature of starch is claimed in U.S. Pat. No. 3,346,563.
European Patent 0,031,477 likewise teaches a method of reacting starch with alkylene epoxides which is carried out in the presence of water, calcium hydroxide or calcium oxide at temperatures of 50.degree. to 120.degree. C.
A method for the cationization of starch is shown in DE-OS 29 35 338 in which, however, the excess alkali must be neutralized.
U.S. Pat. No. 4,281,109 recommends mixing the etherification reagent and the alkali or alkaline earth hydroxide in advance and only then compounding them with the starch.
This procedure is intended to achieve a more even and more rapid intermixing of the reagents so that, for example, local accumulations of alkali do not occur which can result in a swelling of the starch.
All methods cited from the prior art share the common feature that the reaction mixture must be constantly mixed until the conclusion of the etherification reaction.
As the temperature decreases, the reaction time increases. The reaction containers are blocked for a correspondingly long time and agitating energy must be expended.
In addition, yield losses must be accepted.